Testing the Trivers-Willard Hypothesis Using Mothers’ Milk

In poor families, mothers’ milk is richer for daughters than sons: A test of Trivers-Willard hypothesis in agropastoral settlements in Northern Kenya.

Fujita M, Roth E, Lo YJ, Hurst C, Vollner J, Kendell A.

Department of Anthropology, Michigan State University, East Lansing, MI 48824. [email protected]

The Trivers-Willard hypothesis predicts the unequal parental investment between daughters and sons, depending on maternal condition and offspring reproductive potential. Specifically, in polygynous populations where males have higher reproductive variance than females, it predicts that mothers in good condition will invest more in sons, whereas mothers in poor condition will invest more in daughters. Previous studies testing this hypothesis focused on behavioral investment, whereas few examined biological investment. This study investigates the Trivers-Willard hypothesis on both behavioral and biological parental investment by examining breastfeeding frequencies and breast milk fat concentrations. Data from exclusively breastfeeding mothers in Northern Kenya were used to test hypotheses: Economically sufficient mothers will breastfeed sons more frequently than daughters, whereas poor mothers will breastfeed daughters more frequently than sons, and economically sufficient mothers will produce breast milk with higher fat concentration for sons than daughters, whereas poor mothers will produce breast milk with higher fat concentration for daughters than sons. Linear regression models were applied, using breastfeeding frequency or log-transformed milk fat as the dependent variable, and offspring’s sex (son = 1/daughter = 0), socioeconomic status (higher = 1/lower = 0), and the sex-wealth interaction as the predictors, controlling for covariates. Our results only supported the milk fat hypothesis: infant’s sex and socioeconomic status interacted (P = 0.014, n = 72) in their relation with milk fat concentration. The model estimated that economically sufficient mothers produced richer milk for sons than daughters (2.8 vs. 0.6 gm/dl) while poor mothers produced richer milk for daughters than sons (2.6 vs. 2.3 gm/dl). Further research on milk constituents in relation to offspring’s sex is warranted. Am J Phys Anthropol , 2012. © 2012 Wiley Periodicals, Inc.

For another interesting, if highly speculative, take on the Trivers-Willard hypothesis in relation to Mitt Romney and Barack Obama, see this piece at NRO. Note that Romney has five sons and no daughters, and his grandchildren exceed a two to one ratio of boys to girls, about what the hypothesis predicts for a high status man. Obama has two daughters, no sons.


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JP says August 29, 2012

OT: Don’t fear the statins. Yummy, yummy statins. Crush them up and sprinkle them on your breakfast cereal!


Statins fear is ‘putting patients health at risk’, researchers say

Everyone over 50 should consider taking statins to reduce the risk of a heart attack because the possible side effects have been exaggerated, a leading expert has said.

Sir Rory Collins, of Oxford University, said taking cholesterol-lowering statins before warning signs start to appear could provide much more protection from heart attacks or stroke.

He accused medical regulators of overstating the possible sideeffects of statins, the majority of which have not been borne out in clinical trials, because it could encourage them to stop taking the medication and put their health at risk.

He disputed claims that statins can cause sleep disturbances, memory loss, sexual dysfunction, depression, lung disease, cataracts, diabetes, memory loss and confusion.

The only sideeffect proven by experiments is a very low risk of myopathy – a condition which causes muscles to weaken – which is easily outweiged by the benefit to the heart of taking the drugs, Sir Rory said.

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